In the transmission of digital data on a telephone channel, for example, signal degradation generally occurs on account of thermal noise and interference between adjoining data pulses which may partly overlap, especially if transmission speeds are high. To minimize such distortion, use is conventionally made of filtering networks known as equalizers which are designed to flatten the amplitude characteristic and to linearize the phase characteristic of the transmission channel. Such conventional equalizers, consisting of cascaded passive phase shifters, are based upon the structural characteristics of the signal path and cannot take into account certain factors arising only in operation.
More recent developments, therefore, include the design of adjustable equalizers of the so-called "transverse-filter" type with a response characteristic adaptable to existing operating conditions. These equalizers are put through two successive phases, i.e. a preliminary or acquisition phase and a subsequent operational or working phase. In the acquisition phase the equalizer rapidly adjusts itself, on the basis of a series of predetermined test codes transmitted over the channel and identical reference codes generated locally at the receiving end, while during the working phase it follows the gradual changes in transmission characteristics in response to an error signal fed back from a signal-regenerating unit in its output. Such a signal regenerator, which may be referred to as a decision network, quantizes the data pulses issuing from the equalizer according to predetermined levels of pulse magnitude; the detected differences between the quantized levels and the actual output signal serve for the automatic adjustment of the parameters of the equalizer in a sense tending to reduce the error signal to zero.
In the system described and claimed in our above-identified copending application, the equalizer has a data lead for incoming pulses as well as several branch leads each provided with adjustable digital multipliers acting as weighting means. The data lead and the branch leads are connected to a summing circuit which algebraically combines an incoming data pulse with weighted pulses from the several branch leads to form an updating signal fed in parallel to all these leads. The branch leads are further connected to a synthesizing circuit which additively combines their weighted pulses, derived from the updating signal, into a composite signal. Another summer, acting as a comparison circuit, is connected to the synthesizing circuit and to a source of reference signals, specifically to a local code generator during the acquisition phase and to a quantizing decoder during the working phase, for deriving an error signal from the aforementioned reference and composite signals; with the aid of arithmetic means connected to the comparison circuit, this error signal is translated into a control signal fed to the weighting means for adjusting same as to vary the magnitude of the weighted pulses in a sense tending to reduce the error signal. The synthesizing circuit comprises a multiplicity of cascaded delay networks respectively inserted in the several branch leads downstream of their weighting means, each branch lead other than the first one containing an adder at a junction between its multiplier and its delay network connected to the output of the immediately preceding (upstream) delay network.